An ion refers to an element in the periodic table in a certain charge state. All ions refer to all elements in the periodic table in all charge states that the elements can take in principle. Further, the ions include particles consisting of a large number of molecules such as compounds or protein.
An accelerator is a device for accelerating charged particles such as electrons, protons and ions to a high-energy state on the order of several million electron volts (several MeV) to several trillion electron volts (several TeV), and is broadly classified into radio frequency accelerators and induction accelerators, according to acceleration principles. In addition, an accelerator is classified into linear accelerators and circular accelerators according to their geometrical shapes.
The radio frequency circular accelerator is classified into a cyclotron and an rf synchrotron according to acceleration methods. There are radio frequency accelerators of various size according to use; large-sized accelerators for research in nuclear and particle physics that enable obtainment of extremely high energy, and recent small-sized rf synchrotrons for cancer therapy that provide ion beams of a relatively low energy level.
In the radio frequency accelerator, an rf cavity has been used for accelerating charged particles. The rf cavity produces an rf electric field of several MHz to several tens of MHz in synchronization with traveling of the charged particles by resonant excitation of the rf cavity. Energy from the rf electric field is transferred to the charged particles. A resonance frequency is changed within the range described above, because a revolution frequency at which the charged particle circulates around a design orbit increasing with the energy change of the charged particle.
FIG. 10 shows a conventional rf synchrotron complex 34. An rf synchrotron 35 has been particularly an essential tool for experiments in nuclear and high energy physics. The rf synchrotron 35 is an accelerator for increasing the energy of charged particles to a predetermined level by the principles of resonance acceleration, |strong focusing|[SA], and phase stability, and has a configuration described below.
The conventional rf synchrotron complex 34 includes an injection device 15 that accelerates ions generated by an ion source 16 to several percent or several ten percent of the speed of light with an rf linear accelerator 17b, and injects the ions from the rf linear accelerator 17b into the subsequent rf synchrotron 35 using an injector 18 constituted by injection devices such as a septum magnet, a kicker magnet, a bump magnet, or the like, the rf synchrotron 35 that accelerates an ion beam 3 to a predetermined energy level, and an extraction device 19 including an extraction system 20 constituted by various magnets that extracts the ion beam 3 accelerated up to the predetermined energy level from the accelerator ring to an ion beam utility line 21 that is a facility 21a in which experimental devices 21b or the like are placed. The devices are connected by transporting vacuum pipes 16a, 17a and 20a. 
The rf synchrotron 35 includes an annular vacuum duct 4 maintained in a high vacuum state, a bending electromagnet 5 that keeps an ion beam 3 along a design orbit, a focusing electromagnet 6 such as a quadrupole electromagnet placed to ensure strong focusing of the ion beam 3 in the vacuum duct 4 both horizontally and vertically, a radio frequency accelerating device 36 constituted by an rf cavity 36a that applies an rf acceleration voltage to the ion beam 3 in the vacuum duct 4 and accelerates the ion beam 3, and a control device 36b that controls the amplitude and phase of applied radio frequency waves, position monitors 35a periodically placed along the entire circumference for measuring the position of the ion beam 3 in the vacuum duct 4, a steering electromagnet 35b for modifying the orbit of the ion beam 3 (referred to as Closed Orbit Distortion) using position information of the ion beam 3 obtained by the position monitors 35a, a bunch monitor 7 that detects passage of the ion beam 3, or the like.
In the rf synchrotron complex 34 having the above described configuration, the ion beam 3 accelerated up to a certain energy level by the rf linear accelerator 17b and injected into the rf synchrotron circulates along the design orbit in the vacuum duct 4 in an advancing axis direction. If the rf voltage is applied to the rf cavity 36a at this time, the ion beam 3 forms a group of charged particles (hereinafter referred to as a bunch) around a certain phase of the rf voltage (called as acceleration phase) by a focusing force in the propagating direction of ions.
Then, the frequency of the rf voltage applied to the rf cavity 36a is increased in synchronization with an excitation pattern of the bending electromagnet 5 that holds the design orbit of the ion beam 3. Also, the phase of the rf voltage at the bunch center is shifted toward an acceleration phase to increase the momentum of the circulating ion beam 3. The frequency of radio frequency waves must be an integral multiple of the revolution frequency of the ion.
It is known that the relationship of p=eBρ is satisfied, where e is a charge of each particle in the ion beam 3, p is its momentum, B is a magnetic flux density of the guiding magnet, and ρ is a radius of curvature by bending in a magnetic field. Also, magnetic field strength of the quadrupole electromagnet for focusing the ion beam 3 horizontally and vertically is increased in synchronization with the increase in momentum of the ion beam 3. Thus, the ion beam 3 circulating in the vacuum duct 4 is always positioned on a predetermined fixed orbit. This orbit is referred to as a design orbit.
For synchronization between the rate of increase in momentum of the ion beam 3 and the rate of change in magnetic field strength, a method can be used for measuring the magnetic field strength of the bending electromagnet 5 with a magnetic field measuring search coil, generating a discrete control clock (B clock) every change in the magnetic field strength, and determining the frequency of the radio frequency waves based on the B clock.
Without the complete synchronization between the change in magnetic field strength of the bending electromagnet 5 and the change in radio frequency, a revolution orbit radius of the ion beam 3 would decrease or increase, displacing the ion beam 3 from the design orbit to eventually collide with the vacuum duct 4 or the like and be lost. Generally, the accelerator is not perfect. In most cases, there should be perturbations to deform the circulating orbit from the design orbit, such as errors rf voltage amplitude. Thus, the system is configured so that a displacement of the ion beam 3 from the design orbit is measured by the position monitor 8 for detecting a momentum shift, the phase of the rf voltage required for the ion beam 3 to circulate along the design orbit is calculated, and a feedback is applied so that the rf acceleration voltage is applied to the bunch center at a proper phase.
By the rf acceleration voltage, individual ions receive focusing forces in the propagating direction of ions and are formed into a bunch, and circulate in the rf synchrotron 35 while moving forward and backward in the propagating direction of the ion beam 3. This is referred to as the phase stability of the rf synchrotron 35.
FIG. 11 shows confinement and acceleration principles (phase stability) of the bunch by the radio frequency waves in the conventional rf synchrotron 35.
In the confinement method in the advancing axis direction and the acceleration method of the charged particles in the rf synchrotron 35, it is known that a phase space area in which the bunch 3a can be confined is restricted in principle particularly in the advancing axis direction (time axis direction). Specifically, in a time area where the radio frequency waves 37 are at a negative voltage, the bunch 3a is reduced in energy, and in a time area with a different polarity of a voltage gradient, the charged particles diffuse in the advancing axis direction and not confined. In other words, only a time period of the acceleration voltage 37a shown between the dotted lines can be used for accelerating the ion beam 3.
In the time period of the acceleration voltage 37a, the radio frequency waves 37 are controlled to apply an desired constant acceleration voltage 37b to a bunch center 3b. Thus, the particles positioned in a bunch head 3c have higher energy and arrive earlier at the rf cavity 36a than the bunch center 3b does, and thus receive a lower acceleration voltage 37c than the acceleration voltage 37b received in the bunch center 3b and relatively reduce their velocity. On the other hand, the particles positioned in the bunch tail 3d have lower energy and arrive later at the rf cavity 36 than the bunch center 3b does, and thus receive larger acceleration voltage 37d than the bunch center 3b does and relatively increase their velocity. During the acceleration, the particles repeat this process, changing their sitting positions in the bunch head, center, and tail.
A maximum value of an ion beam current that can be accelerated is determined by the size of space-charge forces that is a diffusion force caused by an electric field in the direction perpendicular to the advancing axis of the beam, produced by the ion beam 3 itself. The charged particles in the accelerator receive a force by the focusing magnets and perform motions similar to a harmonic oscillator called betatron oscillation. When the ion beam current exceeds a certain level, the amplitude of the betatron oscillation of the charged particles reaches the size of the vacuum duct 4 and the ion beam is lost. This is referred to as the space-charge limitation.
To be exact, the limitation is made by a maximum value of a local beam current value, that is, a line current density. In the rf synchrotron 35, the bunch center 3b usually has maximum line density, inevitably causing an imbalance in current density between the bunch center 3b and bunch outer edges such as the bunch head 3c and the bunch tail 3d without any particular improvement. Thus, the current density in the bunch center 3b has to be lower than the limitation. This means that the current density in an rf synchrotron is determined by the charge density in the bunch centre.
Specifically, a resonance frequency frf of the rf cavity 36a is written by frf=¼(L·C)1/2 using electric parameters (inductance L and capacity C) of the rf cavity 36a. The inductance is described by L=1·(μ0μ*/2π) log (b/a) using the geometrical parameters (length l, inner diameter a, outer diameter b) and material characteristics (relative permeability μ*) of a magnetic material loaded in the rf cavity 36a. 
A revolution frequency f0 of the particle and the resonance frequency frf of the rf cavity 36a have to always maintain the relationship of frf=hf0 (h: integer) so as to maintain the synchronization with revolution of particles. This is achieved by exciting the magnetic material with an additional current referred to as a bias current and changing an operation point on a B-H curve, and controlling the relative permeability μ*.
Ferrite is generally used as a magnetic material of the rf cavity 36a. Its maximum inductance is obtained when the bias current is around 0 A, and a resonance frequency determined at the operation point is minimum.
In the rf synchrotron 35 designed and constructed exclusively for protons or particular ions, species and charge state can be selected only within a range allowed by a finite variable width of frequency of the rf cavity 36a itself and a radio frequency power amplifier, such as a triode or tetrode, drives the rf cavity.
Thus, in the conventional rf synchrotron 35, once the ion species to be accelerated, an acceleration energy level, and an accelerator peripheral length are determined, a frequency bandwidth of the radio frequency waves 37 is uniquely determined.
FIG. 12 shows the revolution frequency in the rf synchrotron 35 from injection and to end of acceleration for acceleration of various ions with the KEK 500 MeV booster proton synchrotron (hereinafter referred to as KEK 500 MeVPS) by High energy accelerator research organization (hereinafter referred to as KEK). The axis of ordinate represents the revolution frequency (MHz), and the axis of abscissa represents the acceleration time (msec). The KEK 500 MeVPS is an rf synchrotron 35 for protons having a peripheral length of about 35 m.
H (1, 1), U (238, 39) and U (283, 5) represent a proton, a uranium ion (+39), and a uranium ion (+5) respectively, and changes in acceleration frequency thereof are shown in the figure.
The results in FIG. 12 show that, in the rf synchrotron 35 designed for accelerating protons or light ions, heavy ions such as uranium ions cannot be accelerated from a low energy level of an extremely low revolution frequency up to a high energy level. The revolution frequency of ions heavier than protons and lighter than uranium ions (+5) places within a range shown by the double-headed vertical broken arrow.
On the other hand, cyclotrons have been conventionally used as accelerators for accelerating various ions. Like the rf synchrotron 35, the cyclotron also uses an rf cavity 36a as an accelerating device of an ion beam 3. Thus, from the principle limitation in use of radio frequency waves 37, the cyclotron is used only for ions with the same Z/A, where A is the mass number and Z is the charge state of an ion that can be accelerated.
Further, the revolution orbit of the ion beam 3 is held in a uniform magnetic field from a central portion with the ion source 16 to an outermost portion that an extraction orbit is located, and a necessary magnetic field is produced by a bipolar magnet with iron as a magnetic material. However, such a magnet is limited in physical size.
Thus, the maximum value of acceleration energy in cyclotrons constructed heretofore is 520 MeV per nucleon. The weight of iron reaches 4000 tons.
In recent years, an induction synchrotron as a circular accelerator for protons different from the radio frequency accelerator has been proposed. The induction synchrotron for protons is an accelerator that can eliminate the disadvantages of the rf synchrotron 35. Specifically, the induction synchrotron for protons is an accelerator that can contain a large number of protons in an advancing axis direction while maintaining a constant line density at a limit current value or less.
A first feature of the induction synchrotron for protons is that a proton beam can be confined in the advancing axis direction by a pair of positive and negative induced voltages in pulse generated by an induction cell to form a long proton bunch (super-bunch) in the order of μsec.
A second feature is that the confined super-bunch can be accelerated by an induced voltage of a long pulse length generated by a different induction cell.
Specifically, the conventional rf synchrotron 35 is of a functionally combined type that performs confinement and acceleration of protons with common radio frequency waves 37 in an advancing axis direction, while the induction synchrotron is of a functionally separated type that independently performs confinement and acceleration.
An induction accelerating device allows the separation of the confinement and acceleration of protons. The induction accelerating device includes an induction cell for confinement of protons and an induction cell for acceleration of protons as one-to-one transformers having magnetic material cores, and switching power supplies for driving the induction cells, or the like.
A pulse voltage is generated in the induction cell in synchronization with a revolution frequency of a proton beam. For example, in an accelerator having a circumference on the order of 300 m, a pulse voltage has to be generated at a repetition of 1 MHz CW.
As a direct application of the induction synchrotron for protons, a proton driver for exploring next-generation neutrino oscillations and proton-proton colliders employing super-bunches have been proposed. With these accelerators, it is expected that a higher proton beam intensity four times the proton beam intensity of a proton accelerator realized by the conventional rf synchrotron 35 is achieved.
A collider as an application of the induction synchrotron is referred to as a super-bunch hadron collider. The super-bunch hadron collider that makes the most use of the specific features of an induction synchrotron is expected to realize a luminosity an order of magnitude larger than a collider of the same size based on a synchrotron using the conventional radio frequency waves 37. This is equivalent to the luminosity simultaneously provided by 10 colliders based on the rf synchrotron. It is noted that the construction cost of each collider can reach 300 billion yen.
Now, the acceleration principle in the induction synchrotron will be described. Induced voltages having different polarities are generated by the induction cells. A velocity of proton having momentum larger than momentum of an ideal particle positioned in the bunch center 3b is higher than that of the ideal particle, and thus the proton advances and reaches the bunch head 3c. When the proton reaches the bunch head 3c, the proton is reduced in velocity by a negative induced voltage, reduced in momentum, and becomes lower in velocity than the ideal particle locating at the bunch center, and starts moving backward of the bunch 3a. When the proton reaches the bunch tail 3d, the proton starts receiving a positive induced voltage, and is accelerated. Thus, the momentum of the proton exceeds the momentum of the ideal particle. During acceleration, all protons belonging to the proton bunch repeat the above described process.
This is essentially the same as the well-known phase stability (FIG. 11) of the rf synchrotron 35. By this property, the protons are confined in the form of the bunch 3a in the advancing axis direction.
However, the proton cannot be accelerated by induced voltages having the different polarities. Thus, the proton has to be accelerated by another induction cells that can apply a uniform positive induced voltage. It is known and demonstrated that the functional separation of confinement and acceleration significantly increases flexibility in beam handling in the propagating direction of ions.
An induction accelerating device that generates an induced voltage of 2 kV at a repetition rate of 1 MHz CW has been completed and introduced in the KEK 12 GeV proton rf synchrotron (hereinafter referred to as 12 GeVPS). The 12 GeVPS is an rf synchrotron 35 for protons having a circumference of about 340 m. In the recent experiment on induction acceleration where a proton bunch was confined by the existing rf voltage and accelerated with the induction voltage, the 12 GeVPS has succeeded to demonstrate the induction acceleration of a proton beam from 500 MeV up to 8 GeV.
The above demonstrated technique is described in “The Physical Society of Japan, Vol. 59, No. 9 (2004), p601-610, Phys. Rev. Lett. Vol. 94. No. 144801-4 (2005)”.
However, it has been heretofore considered to be impossible to accelerate various species of ion in their allowed charge states in a single accelerator to obtain high energy.
This is because in the conventional rf synchrotron 35, the rf cavity 36a as a resonator used for acceleration has a high quality factor, and radio frequency waves 37 can be excited only in a finite band width. Thus, when the circumference of the rf synchrotron 35, the field strength of the bending electromagnet 5 used, and the bandwidth of the radio frequency waves 37 used are determined, the mass number A and the charge state Z of ions that can be accelerated are substantially and uniquely determined and only the limited ions can be accelerated in a low energy area where the velocity significantly changes.
On the other hand, in a cyclotron, only ions having a constant ratio between the mass number and the charge state can be accelerated correspondingly to the bandwidth of the radio frequency waves 37. Also, in an electrostatic accelerator such as a Van de Graaff accelerator that can accelerate any ions, the limit of acceleration energy is 20 MeV from the capability of voltage-resistance of the device in vacuum or pressured gas.
The linear induction accelerator can provide a energy of several hundred MeV or more, but the cost for obtaining the energy and the physical size of the linear induction accelerator become enormous. Parameters of the linear induction accelerator presently obtained are substantially a hundred million yen/1 MeV and 1 m/1 MeV. Thus, obtaining an ion beam of 1 GeV requires a cost of 100 billion yen, and the entire length of the accelerator of 1 km.
Further, in the induction synchrotron for protons, such as the KEK 12 GeVPS that has been demonstrated as an induction synchrotron, its injection energy is already sufficiently high, and acceleration of protons substantially having the speed of light only has been considered. Specifically, the proton beam is already accelerated substantially up to the speed of light in the upstream accelerator. Thus, when the protons are accelerated by the induction synchrotron, it is only necessary to generate an induced pulse voltage of the induction cell at almost constant intervals. Thus, trigger timing of the induced voltage applied to the proton beam needs not to be changed with acceleration.
However, when all ions are accelerated in a single induction synchrotron, the trigger timing of the induced voltage has to be changed depending on the revolution of individual ion species. This is because the revolution frequency significantly differs among ion species as shown in FIG. 12.
Thus, the present invention has an object to provide an accelerator that can accelerate by itself all ions up to any energy level allowed by the field strength of electromagnets used for beam guiding (hereinafter referred to as any energy level).